1. Field of the Invention
The present invention relates to an unevensurface data detection apparatus used for registering uneven-surface data such as a human fingerprint and collating a human fingerprint with uneven-surface data which has already been registered.
2. Description of the Related Art
In the field of a criminal investigation, there is provided a collation system in which a human fingerprint or palm-print is collated with data so that a suspect can be identified. In this collation system, a human fingerprint or palm-print is usually processed as an image. Therefore, it is necessary to provide an input device by which a human fingerprint or palm-print is converted into image data. As an electrical means for directly picking up a human fingerprint, there is a method shown in FIG. 19. When a finger 10 is pressed against one surface 31a of the transparent parallel plate 31, which is a light guiding plate, a projecting portion 10a of the finger 10 comes into contact with the plane 31a, however, a recess portion 10b does not come into contact with the plane 31a. When light sent from LED 33 is irradiated on the plane 31a, against which the finger is pressed, via the transparent base 31, it is reflected on the surface of the finger and scattered inside the finger.
Scattered light sent from the recess portion 10b of the finger 10 passes through an air layer and is incident on the transparent base 31. Accordingly, there is no component of light which is totally reflected and propagated in the transparent base 31. However, light reflected and scattered by the projecting portion 10a is directly incident on the transparent base 31 as a spherical wave, and a portion of the light satisfies the total reflecting condition in which light is totally reflected in the transparent base 31, and light propagates in the transparent base 31 while the total reflection is repeated. When this total reflection component is focused on CCD 37 by an appropriate optical system including a mirror 32, diaphragm 34, lense 35 and mirror 36, it is possible to obtain an image of the ridge pattern of the projecting portion 10a in real time.
However, the following problems may be encountered in the above optical system. A fingerprint image outputted from the above optical system is distorted trapezoidally as shown in FIG. 2, that is, the magnification of x and Y-directions are different from each other. There are two reasons which will be described as follows. One reason is a difference in the image forming magnification caused by a difference in the length of the optical path between the input surface and the image forming system, which originates from the input surface, which is oblique to the optical axis as shown in FIG. 4. The other reason is a difference in the magnifications of X and Y-directions of the formed fingerprint image which is caused by the oblique input surface shown in FIG. 5.
In order to solve the above problems, Japanese Unexamined Patent Publication No. 62-187819 discloses a method in which light emitted from the transparent flat plate is guided by hologram. However, the above technique has the following disadvantages. When a hologram is used, the angle of diffraction is sensitive to the wavelength of the light source. Therefore, it is necessary to use a light source such as a laser, the wavelength region of which is small, therefore the resolution of the image is deteriorated due to astigmatism. Another method is disclosed in Japanese Unexamined Patent Publication No. 2-176984, in which an optical path length compensating prism is provided for compensating for the optical path length on the transparent flat plate. However, the above method is disadvantageous in that the magnification of X and Y-directions can not be sufficiently compensated for by only the compensating prism in the image formation. For the above reasons, there is caused a problem that a square image is detected as a rectangular image as shown in FIG. 20. In this case, it is necessary to convert the resolution of X and Y-directions when the image is processed. In the case of picking up a finger print used as a piece of evidence for the purpose of a criminal investigation, it is preferable that image processing is not conducted. Accordingly, it is preferable to provide a fingerprint image pickup apparatus in which all image compensation can be conducted in the optical system.